Some devices, such as integrated circuit chips, need to be precisely fabricated. Accordingly, inspection of the devices is necessary to ascertain whether the devices meet exacting acceptance standards. The devices to be inspected are often provided in compartmented trays that have multiple rows and columns of pockets in which the devices are transported.
An inspection and handling system is utilized to inspect such devices. Trays of devices are transported through various stages of the inspection and handling system including laser scanning, inversion, camera scanning, and individual placement at a final destination so that devices meeting the exacting acceptance standards are separated from those devices which do not meet such standards.
One final destination of devices that meet the acceptance standards is carrier tape. Typically, carrier tape is an elongated tape that includes pockets that are arranged in series. The pockets are typically shaped to be complementary to the dimensions of the devices that are to be housed therein. An instrument such as a vacuum operated precisor of a pick and place system can transport a device from a tray into a pocket of the carrier tape. Once devices are individually placed into the pockets of the carrier tape, a cover tape is often applied and the carrier tape with devices housed in the sealed pockets can be wound onto a reel and conveniently transported to another destination, such as on in which the devices will be put into final use.
Pick and place systems are generally capable of motion in one direction (transverse to the direction of movement of trays through the inspection and handling system) and have limited, if any, movement in a direction perpendicular to that motion (parallel to the direction of movement of the trays). Therefore, the carrier tape needs to be incrementally moved by a drive system so that the pick and place system can place devices into successive pockets of the carrier tape. It is therefore necessary to determine the location of individual pockets of the carrier tape with respect to the pick and place system. Typically, the carrier tape includes sprocket holes that run the length of the carrier tape on one or both sides of the pockets. The sprocket holes are utilized to determine the position of a pocket relative to the pick and place system.
In some inspection and handling systems, problems may occur if the carrier tape is not consistently advanced by the proper distance equivalent to the length of one pocket. Traditionally, the beginning of a pocket is determined by forwarding the carrier tape by a fixed distance, and assuming that the carrier tape moved forward the distance programmed. Use of a sensor may also be employed to detect the number of sprocket holes passed as the carrier tape is advanced. In such systems, the drive system assumes that the point to which the carrier tape is forwarded is the correct starting point of the pocket.
Specifically, in some systems, simply advancing the carrier tape by a set distance, or counting the number of sprocket holes passed, may not be sufficiently accurate in determining the position of a pocket. Jitter or slipping of the carrier tape can occur. In the case of slipping, the carrier tape may not be advanced the amount the drive system is programmed to advance. In the case of jitter, the carrier tape may move backward, thereby counting a sprocket hole twice. Accordingly, errors may incur in determining the location of a pocket.